"What does .class.class do" It selects elements with the class `class` that also have the class `class`. The example might be a little abstract but it could be

"What does .class.class do" It selects elements with the class `class` that also have the class `class`. The example might be a little abstract but it could be something like `.message.highlight`, which selects all elements with the class `message` that also have the class `highlight`. Classes are for classification, IDs are identification. For styling you almost never need identifiers because that is not what styling is about. <div class="message highlight"> .message.highlight {} is much better than <div id="highlightMessage"> #highlightMessage {} IDs are like singletons: not reusable.